One of the most important aspects to consider in the control of grid-connected devices (such as controlled AC-DC converters, active filters, cycloconverters, and other energy storage systems coupled with the utility voltage) is the proper synchronization with the reference grid. The most widely accepted solution to provide this synchronization are the PLL technics. In general, a conventional PLL comprises a phase frequency detector (PFD) aimed to detect phase error and frequency error between the sensed input signal and an internally reconstructed signal. This error signal is processed in a filter and the result is used to adjust the frequency of a voltage controlled oscillator (VCO). This process continues until the angular frequency and phase angle of the internally reconstructed signal match those of the input signal. A PLL of the generic type is specified in the U.S. Pat. No. 6,008,703, which is titled “Digital compensation for wideband modulation of a phase locked loop frequency synthesizer” by M. Perrott, et al., which is incorporated by reference.
In most conventional control strategies for power electronic systems, e.g. active filters, active rectifiers, etc., both the magnitude and phase angle of the positive-sequence voltage signal are necessary. These quantities are mainly used for synchronization of the system output variables, power flux calculations or for the transformation of state variables into rotating reference frame coordinates. Regardless of the technique used in the detection process, the amplitude and the phase angle estimation of the positive-sequence component must be computed as fast and as accurate as possible despite of the unavoidable disturbances in the electric utility. Among the main disturbances are voltage harmonic distortion, voltage unbalance and angular frequency variations.
The most extended technique used for frequency-insensitive positive-sequence detection is the conventional three-phase PLL based on the synchronous reference frame (SRF-PLL). In the conventional SRF-PLL, the three-phase voltage vector is translated from the natural reference frame to the rotating reference frame by using a combination of Clarke's and Park's transformations. The angular position of this reference frame is controlled by a feedback loop which regulates the q-component of the positive sequence of the voltage to zero. Therefore in steady-state, the detected d-component depicts the voltage vector amplitude, while its phase angle is determined by the output of the feedback loop. Under ideal utility conditions, i.e., neither harmonic distortion nor unbalance is present, a high band-width feedback loop of the SRF-PLL yields a fast and precise detection of the phase angle and amplitude of the utility voltage vector. In case the utility voltage is distorted with high-order harmonics, the bandwidth of the SRF-PLL feedback loop can be reduced to achieve a satisfactory operation, i.e., to reject and cancel out the effect of the harmonics on the output. However, the PLL bandwidth reduction is not an acceptable solution in the presence of unbalanced grid voltages.
The operation of the conventional SRF-PLL is reported in V. Kaura and V. Blasco, “Operation of a phase locked loop system under distorted utility conditions,” IEEE Trans. on Ind. Electr., Vol. 33, No. 1, pp. 58-62, January/February 2007; and in P. Rodríguez, et al., “Decoupled double synchronous reference frame PLL for Power converters control,” IEEE Trans. on Ind. Electr., Vol. 22, No. 2, pp. 584-592, March 2007, which are incorporated by reference.
Other possibilities for the positive-sequence detection are based on the instantaneous symmetrical components (ISC), on space vector filters (SVF), and on the recursive weighted least-square estimation algorithm (WLSE), which are reported in A. Ghosh, et al., “A new algorithm for the generation of reference voltages of a DVR using the method of instantaneous symmetrical components,” IEEE Power Eng. Rev., Vol. 22, No. 1, pp. 63-65, January 2002; J. Svensson, “Synchronisation methods for grid connected voltage source converters,” Proc. Inst. Electr. Eng., Vol. 148, No. 1, pp. 229-235, May. 2001; and H. Song, et al., “An instantaneous phase angle detection algorithm under unbalanced line voltage condition,” Proc. IEEE Power Electron. Spec. Conf., Vol. 1, pp. 533-537, August 1999.